We report on the characterization of selectively doped GaAs/AlGaAs heterostructures, grown by an extremely clean molecular beam epitaxy system, which exhibit a Hall mobility of a two dimensional electron gas exceeding10×106 cm2/Vsfor a wide range of undoped spacer layer thickness (50–100 nm). A maximum electron mobility of14.4×106 cm2/Vswas measured at 0.1 K in a structure with a 68 nm spacer thickness and an areal carrier density of2.4×1011cm−2. This is the highest electron mobility ever reported, leading to a momentum relaxation mean-free path of∼120&mgr;m. We present experiments that enable us to distinguish between the main scattering mechanisms. We find that scattering due to background impurities limits electron mobility in our best samples, suggesting that further improvement in structure quality is possible. ©1997 American Institute of Physics.